Set the oven's temperature to 450°F (230°C).
Use parchment paper to cover a baking sheet.
Remove the leaves from the cauliflower after washing it.
Remove the core and about three-quarters of the stem, leaving just enough stem to support the cauliflower upright.
Place the entire cauliflower in a big pot, fill it with water, and cook it to a boil over high heat.
Carefully add the cauliflower when the water is boiling, and cook for 5 to 7 minutes (5 minutes if the cauliflower is smaller, 7 minutes if it is bigger).
Carefully remove the cauliflower from the pot using a pair of tongs.
Place it stem-down on the baking sheet after allowing it to drain for a short while.
To roast for 25 to 30 minutes, brush with olive oil, season with salt and za'atar, and roast until the outside is crispy (but not burnt), and the inside is soft.
To see if it is finished, insert a fork into the stem.
Put the cauliflower under the broiler for about 5 minutes if it isn't nicely browned.
Before serving, let it cool down a little.